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Gas gain stabilisation in the ATLAS TRT detector

Mindur, B.; Akesson, T. P. A.; Anghinolfi, F.; Antonov, Anton; Arslan, O; Baker, O. K.; Banas,E.; Bault, C.; Beddall, A. J.; Bendotti, J.; Benjamin, D. P.; Bertelsen, H.; Bingul, A.; Bocci, A.;Boldyrev, A. S.; Brock, I.; Garrido, M. Capeans; Catinaccio, A.; Celebi, E.; Cetin, S. A.; Choi,John K; Dam, Mogens; Danielsson, H.; Davis, D. R.; Degeorge, C.; Derendarz, D.; Desch, K.;Di Girolamo, B.; Dittus, F.; Dixon, Nicholas E.; Dressnandt, N.; Dubinin, F. A.; Evans, DavidH; Farthouat, P.; Fedin, O. L.; Froidevaux, D.; Gavrilenko, I. L.; Gay, C.; Gecse, Z.;Godlewski, J.; Grefe, C.; Gurbuz, S.; Hajduk, Z.; Hance, M.; Haney, B.; Hansen, JørgenBeck; Hansen, P. H.; Hawkins, A. D.; Heim, S.; Holway, K.; Kantserov, V. A.; Katounine, S.;Kayumov, F.; Keener, P. T.; Kisielewski, B.; Klopov, N. V.; Konovalov, S. P.; Koperny, S.;Korotkova, N. A.; Kowalski, T. Z.; Kramarenko, V.; Krasnopevtsev, D.; Kruse, M.; Kudin, L.G.; Lichard, P.; Loginov, A.; Martinez, N. Lorenzo; Lucotte, A.; Luehring, F.; Lytken, E.;Maleev, V. P.; Maevskiy, A. S.; Ramos, J. Manjarres; Mashinistov, R. Y.; Meyer, C.;Mialkovski, V.; Mistry, K.; Mitsou, V. A.; Nadtochi, A. V.; Newcomer, F. M.; Novodvorski, E.G.; Ogren, H.; Oh, S. H.; Oleshko, S. B.; Olszowska, J.; Ostrowicz, W.; Palacino, G.;Patrichev, S.; Penwell, J.; Perez-Gomez, Francisco; Peshekhonov, V. D.; Rohne, O.; Reilly,M. B.; Ricken, O.; Rousseau, D.; Shmeleva, A. P.; Shulga, E.; Sivoklokov, S.; Smirnov, S.;Smirnov, Yu.; Smirnova, L. N.; Soldatov, E.; Sulin, V. V.; Tartarelli, G.; Taylor, W.; Thomson,E.; Tikhomirov, V. O.; Tipton, P.; Valls Ferrer, J. A.; van den Berg, Ria; Vasquez, Juan Luis;Vasilyeva, L. F.; Vlazlo, O.; Weinert, B.; WILLIAMS, H.; Wong, [No Value]; Zhukov, K. I.;Zieminska, D.Published in:Journal of Instrumentation

DOI:10.1088/1748-0221/11/04/P04027

Publication date:2016

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Gas gain stabilisation in the ATLAS TRT detector

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2016 JINST 11 P04027

Published by IOP Publishing for Sissa Medialab

Received: March 21, 2016Accepted: April 21, 2016

Published: April 29, 2016

Gas gain stabilisation in the ATLAS TRT detector

The ATLAS TRT collaborationB. Mindur,h,1 T.P.A. Åkesson,n F. Anghinolfi, f A. Antonov,q O. Arslan,e O.K. Baker,y

E. Banas,i C. Bault, f A.J. Beddall,a J. Bendotti, f D.P. Benjamin,j H. Bertelsen,g A. Bingul,b

A. Bocci,j A.S. Boldyrev,r I. Brock,e M. Capeáns Garrido, f A. Catinaccio, f E. Celebi,c

S.A. Cetin,d K. Choi,l M. Dam,g H. Danielsson, f D. Davis,j C. Degeorge,l D. Derendarz,i

K. Desch,e B. Di Girolamo , f F. Dittus, f N. Dixon, f N. Dressnandt,u F.A. Dubinin,p H. Evans,l

P. Farthouat, f O.L. Fedin,v D. Froidevaux, f I.L. Gavrilenko,p C. Gay,x Z. Gecse,x

J. Godlewski,i C. Grefe,e S. Gurbuz,c Z. Hajduk,i M. Hance,u B. Haney,u J.B. Hansen,g

P.H. Hansen,g A.D. Hawkins,n S. Heim,u K. Holway,j V.A. Kantserov,q S. Katounine,v

F. Kayumov,p P.T. Keener,u B. Kisielewski,i N.V. Klopov,v S.P. Konovalov,p S. Koperny,h

N.A. Korotkova,r T.Z. Kowalski,h V. Kramarenko,r D. Krasnopevtsev,q M. Kruse,j

L.G. Kudin,v P. Lichard, f A. Loginov,y N. Lorenzo Martinez,l A. Lucotte,k F. Luehring,l

E. Lytken,n V.P. Maleev,v A.S. Maevskiy,r J. Manjarres Ramos,z R.Y. Mashinistov,p C. Meyer,u

V. Mialkovski,m K. Mistry,u V.A. Mitsou,w A.V. Nadtochi,v F.M. Newcomer,u

E.G. Novodvorski,v H. Ogren,l S.H. Oh,j S.B. Oleshko,v J. Olszowska,i W. Ostrowicz,i

G. Palacino,z S. Patrichev,v J. Penwell,l F. Perez-Gomez, f V.D. Peshekhonov,m O. Røhne,t

M.B. Reilly,u C. Rembser, f O. Ricken,e A. Romaniouk,q D. Rousseau,s V. Ryjov,m

U. Sasmaz,b S. Schaepe,e V.A. Schegelsky,v A.P. Shmeleva,p E. Shulga,q S. Sivoklokov,r

S. Smirnov,q Yu. Smirnov,q L.N. Smirnova,r E. Soldatov,q V.V. Sulin,p G. Tartarelli,o

W. Taylor,z E. Thomson,u V.O. Tikhomirov,p P. Tipton,y J.A. Valls Ferrer,w R. Van Berg,u

J. Vasquez,y L.F. Vasilyeva,p O. Vlazlo,n B. Weinert,l H.H. Williams,u V. Wong,x K.I. Zhukovp

and D. Zieminskal

aBahcesehir University, Faculty of Engineering and Natural Sciences, 34353, Besiktas, Istanbul, TurkeybGaziantep University, Department of Physics Engineering, 27300, Sehitkamil, Gaziantep, TurkeycBogazici University, Department of Physics, 34342, Bebek, Istanbul, TurkeydIstanbul Bilgi University, Faculty of Engineering and Natural Sciences, 34060, Eyup, Istanbul, TurkeyePhysikalisches Institut, University of Bonn, Bonn, GermanyfCERN, CH - 1211 Geneva 23, Switzerland, SwitzerlandgNiels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK - 2100 Kobenhavn 0, DenmarkhFaculty of Physics and Applied Computer Science of the AGH-University of Science and Technology,(FPACS, AGH-UST), al. Mickiewicza 30, PL-30059 Cracow, Poland

1Corresponding author.

© CERN 2016 for the benefit of the ATLAS collaboration, published under the terms ofthe Creative Commons Attribution 3.0 License by IOP Publishing Ltd and SissaMedialab

srl. Any further distribution of this work must maintain attribution to the author(s) and the publishedarticle’s title, journal citation and DOI.

doi:10.1088/1748-0221/11/04/P04027

2016 JINST 11 P04027

iHenryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences,ul. Radzikowskiego 152, PL - 31342 Cracow, PolandjDuke University, Department of Physics, Durham, NC 27708, U.S.A.kLaboratoire de Physique Subatomique et de Cosmologie, CNRS-IN2P3, Universite Joseph Fourier, INPG,53 avenue des Martyrs, FR - 38026 Grenoble Cedex, FrancelIndiana University, Department of Physics,Swain Hall West, Room 117, 727 East Third St., Bloomington, IN 47405-7105, U.S.A.

mJoint Institute for Nuclear Research, JINR Dubna, RU - 141 980 Moscow Region, RussianLunds Universitet, Fysiska Institutionen, Box 118, SE - 221 00 Lund, SwedenoINFN Milano and Università di Milano, Dipartimento di Fisica, via Celoria 16, IT - 20133 Milano, ItalypP.N. Lebedev Institute of Physics, Academy of Sciences, Leninsky pr. 53, RU - 117 924 Moscow, RussiaqNational Research Nuclear University MEPhI, Kashirskoe Shosse 31, RU -115409 Moscow, RussiarLomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics,RU - 119 992 Moscow Lenskie gory 1, Russia

sLAL, Univ. Paris-Sud, IN2P3/CNRS, Orsay, FrancetDepartment of Physics, University of Oslo, Blindern, NO - 0316 Oslo 3, NorwayuUniversity of Pennsylvania, Department of Physics & Astronomy,209 S. 33rd Street, Philadelphia, PA 19104, U.S.A.

vPetersburg Nuclear Physics Institute, RU - 188 300 Gatchina, RussiawInstituto de Física Corpuscular (IFIC), Centro Mixto UVEG-CSIC, Apdo. 22085, ES-46071 Valencia;Dept. Física At., Mol. y Nuclear, Univ. of Valencia and Instituto de Microelectrónica de Barcelona(IMB-CNM-CSIC), 08193 Bellaterra, Barcelona, Spain

xUniversity of British Columbia, Department of Physics,6224 Agriculture Road, CA - Vancouver, B.C. V6T 1Z1, Canada

yYale University, Department of Physics, PO Box 208121, New Haven CT, 06520-8121, U.S.A.zYork University, Department of Physics and Astronomy, Toronto ON , Canada

E-mail: bartosz.mindur@agh.edu.pl

Abstract: The ATLAS (one of two general purpose detectors at the LHC) Transition RadiationTracker (TRT) is the outermost of the three tracking subsystems of the ATLAS Inner Detector. Itis a large straw-based detector and contains about 350,000 electronics channels. The performanceof the TRT as tracking and particularly particle identification detector strongly depends on stabilityof the operation parameters with most important parameter being the gas gain which must bekept constant across the detector volume. The gas gain in the straws can vary significantly withatmospheric pressure, temperature, and gas mixture composition changes. This paper presents aconcept of the gas gain stabilisation in the TRT and describes in detail the Gas Gain StabilisationSystem (GGSS) integrated into the Detector Control System (DCS). Operation stability of the GGSSduring Run-1 is demonstrated.

Keywords: Gaseous detectors; Particle tracking detectors (Gaseous detectors); Transition radiationdetectors; Wire chambers (MWPC, Thin-gap chambers, drift chambers, drift tubes, proportionalchambers etc)

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Contents

1 Introduction 1

2 Gas gain stabilisation in the ATLAS TRT 3

3 Operation principles of the Gas Gain Stabilisation System 5

4 Properties of the reference straw tubes and related components 74.1 Gas gain measurements 74.2 Gas gain temperature dependence 9

5 Design of the Gas Gain Stabilisation System 95.1 Hardware 95.2 Software 11

6 Longterm system performance 126.1 Reference high voltage 126.2 Gas gain stability 13

7 Conclusions 14

1 Introduction

The ATLAS Inner Detector (ID) is composed of three detector sub-systems: the silicon-based Pixeland SemiConductor Tracker (SCT) detectors, and the gaseous drift tube (straw) based TransitionRadiation Tracker (TRT) [1]. Schematically the design of the ATLAS Inner detector is shown infigures 1 and 2.

The TRT is the outermost of the three sub-systems (see [2–4] for more details). The activeregion of the TRT detector contains 298,304 straw drift tubes of 4mm diameter (350,848 electronicschannels). The barrel section of the TRT covers 560 < R < 1080 mm and |z| < 720 mm and hasthe straws aligned parallel to the direction of the beam axis. The two end-cap sections cover827 < |z| < 2744 mm and 617 < R < 1106 mm and have the straws arranged radially in wheels.

The TRT exploits a novel design which combines continuous tracking capability with particleidentification (PID) based on transition radiation (TR). The latter functionality provides substantialdiscriminating power between electrons and pions over the energy range between 1 and 100GeVand is a crucial component of the ‘tight’ electron selection criteria in ATLAS. The TRT straws arefilled with a gas mixture of 70% Xe, 27% CO2 and 3% O2. Xenon is used for its high efficiency toabsorb TR photons of typical energy 6–15 keV. The space between the straws is filled with radiatormaterial. The TR photons (soft X-rays) emitted in the radiator are absorbed in the gas inside thestraw tubes, which serve as detecting elements both for tracking and for particle identification (more

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Figure 1. Schematic view of the Barrel part of the ATLAS Inner detector.

Figure 2. Schematic view of the End Cap (EC) part of the ATLAS Inner detector.

about TRT performance can be found in [5]). The straw wall is held at a potential of about -1530 Vwith respect to a 31 µm diameter gold-plated tungsten wire at the centre that is referenced to ground.The electrons drift towards the wire and cascade in the strong electric field very close to the wirewith a coefficient of 2.5 × 104 (gas gain), thus producing a detectable signal. The signal in eachwire is amplified, shaped and discriminated against two adjustable thresholds: the Low Level (LL)

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threshold (∼300 eV), is used to calculate the track to anode wire distance by measuring the drifttime of the closest electron cluster to the wire and the High Level (HL) threshold (∼6 keV) used toidentify a large energy deposits from absorbed TR photons for particle identification.

The low level threshold is set to a minimum defined by the rate of electronics noise pulses.Any increase of this threshold leads to a worsening of the drift time accuracy measurement. Forinstance, an increase of the LL threshold by 20% leads to a change of a coordinate accuracy from125 µm to 130 µm and to a reduction of drift time measurement efficiency by 5% [6].

Particle separation is based on probability to exceed the high level threshold. This probabilityis different for particles which produce TR (Lorentz γ factor above ∼500) and other particles witha γ factor below ∼500. This probability is highly sensitive to the value of the threshold. It hasbeen shown that TRT particle separation power stays the same if the variation of the effectivethreshold across the detector is stable to within 20% [5]. This constraint seems loose but it mustbe maintained against all possible uncertainties like signal amplitude variations along the straws,electronics channel to channel gain variations, gas gain variations etc. Some of these sources of am-plitude variations are quite stable over time and can be partially compensated by regular calibrationprocedures. However there are other sources of amplitude variation for which the environmentalchange is too short of a time (hours) to be compensated by an offline compensation procedure. Thegas gain is sensitive to three such varying parameters: the gas pressure, gas temperature and gascomposition. Since these parameters can and do change over the course of a run, a real-time hard-ware compensation system to adjust the high voltage on the straws called the Gas Gain StabilisationSystem was developed. Apart from global variations of temperature and pressure there are localvariations of temperatures within the TRT detector volume which also must be compensated.

During the Large Hadron Collider (LHC) collisions the ionisation currents generated withinstraws are sufficiently large such that they heat the gas inside the straws and temperature within thedetector becomes dependent on the LHC luminosity leading to an unavoidable time dependenceof the gas gain of the course of each run. The design of the ATLAS inner detector assumedthermal neutrality of neighbouring subdetectors. That is, the design tried to keep the inevitableheat production (e.g. electronics, ionisation in straws) confined within the originating subsystemuntil it could be removed by a cooling system. Thermal barriers between the TRT, which run atroom-temperature and the silicon detectors, which run well below 0C, were foreseen by designbut do not guarantee a uniform temperature distribution across the TRT detector volume. In orderto compensate the effect of the temperature on the gas gain, each partition of the TRT (2 barrel, 2end-cap) is equipped with 236 temperature sensors. Data from these sensors is used to adjust theTRT high voltages in real time.

This paper describes in detail the concept and implementation of the TRT gas gain stabilisation.The design requirement to control the gas gain time variation in all TRT straws is determined suchthat the effective threshold variation is below ∼5%.

2 Gas gain stabilisation in the ATLAS TRT

The ATLAS TRT Gas Gain Stabilisation System maintains gas gain stability throughout the TRTby using the Detector Control System (DCS) [7, 8, 13] to adjust in real time the high voltage(HV) in ∼2000 TRT HV partitions. The GGSS produces the single reference voltage (HVre f ) thatcompensates for variations in the actual composition of active gas as well as its temperature and

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pressure. The TRT has a completely closed loop active gas system which constantly adjusts the gascomposition sent to the detector. The GGSS draws sample gas directly from the TRT gas systemclosed loop return manifold guaranteeing that the HV adjustments are based on the actual flowcondition of the active gas. The GGSS operates at atmospheric pressure while the gas pressure inthe TRT is regulated to be within a few tens of mbar above atmosphere. Since the working gascomposition and pressure are essentially the same within the GGSS and throughout the entire TRT,only variations in temperature across the detector need to be taken into account when calculatingthe high voltage settings for different regions of the detector. The number of temperature sensorsused for stabilisation purposes is far less than the number of HV partitions, hence, the detectorhas to be divided into regions which are chosen according to the sensor topology and mechanicalgranularity (see figure 3).

The TRT Barrel is divided into 4 φ sectors (±45) and into the 3 layers of barrel modules andthus, the barrel contains 12 regions where the HV is regulated independently following temperaturemeasurements. Each of these 12 regions has 3 temperature sensors along Z near the outside radiusof the region. The average of these 3 measurements is taken as ’regional temperature’ used for HVcorrection (see also figure 3a).

The TRT end-caps form stacks of wheels that for stabilisation purposes are divided in φ into 4sectors (±45) and into the number of wheels in Z (14) and thus each end-cap contains 56 regions(see figure 3b). Each of these regions have sensors on their front an back in Z and the average ofthese two sensors is taken as representative for the region.

(a) Regions in barrel part. (b) Regions in endcap part.

Figure 3. Detector segmentation used for the TRT gas gain stabilisation based on the detector temperatureinformation.

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The temperature inside the GGSS system is quite stable and measured with an accuracy ofabout 0.1 C, while the temperature variations within the TRT detector are much larger. Within anhour of powering the electronics, the temperatures across the detector start to stabilise. However, atdifferent regions of the TRT the temperature variation is significant (6-9C). During the course of arun, regional temperatures are varying depending on local particles fluence and amount of ionisationdeposition hence the variations must be compensated for in real time. The single reference voltagevalue produced by GGSS system is sent to the TRT DCS HV controllers where the HV for eachdetector region is recalculated and set according to the following formula:

HVcor = HVref + α(Treg − Tggss) (2.1)

where: HVcor is the voltage to be applied in region, HVref reference voltage, α is thermal coefficientof the active gas and Treg and Tggss are the regional and the GGSS temperatures respectively. Toprotect the TRT sensors (i.e. straws) against failures of the GGSS and dangerous environmentalconditions, the TRT HV DCS system has several protection mechanisms. The calculated setting iscompared to amaximum voltage value protecting the detector against overvoltage. If an overvoltagesetting is requested, the HV is set to the maximal value. The GGSS also sends HVref quality flagsafter every measurement cycle to provide information whether voltage value is to be trusted or not.If any of these flags are true, the TRT HV is not changed.

3 Operation principles of the Gas Gain Stabilisation System

GGSS operation is based on a stabilisation of the signal amplitude produced by 55Fe X-rays inreference (test) straws. The operation of proportional gaseous counter is thoroughly explained inbasic textbooks and the key parameter governing this is the value of the gas gain A, (A = n/n0)(where n0 is number of primary ion pairs liberated in active gas). Changes in the peak position ofthe pulse height spectrum of the 5.9 keV X-ray emission line of a 55Fe source irradiating referencestraws are interpreted as changes in the gas gain. The high voltage on the reference straws is adjustedto maintain the fitted peak at a fixed average value that defines the design gas gain value. After eachmeasuring cycle, the reference voltage is recalculated to maintain the design gas gain and then sentto the DCS which then adjusts the high voltage in each TRT region taking into account the localtemperature. The steps by which the high voltage is modified to maintain the nominal gas gain areas follows (see also figure 4):

• the starting values for the high voltage (HVs) and peak position (pps) are manually entered toinitialise the GGSS when it is turned on (the default values are selected to be typical valuesfor the TRT straws),

• the spectrum of 55Fe is collected for the currently set HV ,

• peak position — ppc, energy resolution, and skewness are calculated using a Gaussian fit ofthe spectrum,

• bad measurements are rejected using the calculated energy resolution and skewness,

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Start stabilization process

Initialize parameters:HVs, pps, Δpp, DV

Stop operation

Setting defaultHV = HVref = HVs

Setting HV = HVref

Collect spectrumand determine ppc

pps - ppc ≤ Δpp

HVref = HV + DV · (pps - ppc) / pps

Continue operation

HVref = const

No Yes

No

Yes| |

Figure 4. Simplified block diagram of the GGSS operation.

• once a good measurement is obtained, the new HV value is calculated from the formula:

HV = HVs +pps − ppc

ppsDV (3.1)

where DV is the voltage needed to double the gas gain (i.e. 70V),

• the new value of the HV is applied to the test straw and the procedure is repeated until thecondition | pps − ppc | ≤ ∆pp is met (where: ∆pp is the required accuracy of peak positionstabilisation, in this case is equal to 10% of pps),

• when | pps − ppc | ≤ ∆pp is achieved, the procedure than sets HVref = HV , where HVrefis the voltage sent to DCS. To ensure robustness of the HVref value it is calculated as themean, HV , of the values obtained in each of the reference straws (using only straws withgood values).

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4 Properties of the reference straw tubes and related components

Stable operation of the GGSS is a key factor for successful TRT operation. Significant work hasbeen carried out to ensure good knowledge of the performance of the basic components of thesystem and study their properties. The straw tubes used as reference detectors have been carefullyexamined and were selected for the uniformity of their response. The following parameters weremeasured in selecting the final straw tubes used in the GGSS:

• the gas gain as a function of high voltage and position of radiation source along the straws,

• the counting characteristics (i.e plateau curves),

• the gas gain response to pressure and temperature variations,

• the energy resolution, measured as 55Fe spectrum peak width,

• the long-term stability,

• minimal change in straw tube operation from radiation-induced ageing.

4.1 Gas gain measurements

The gas amplification (gain) is a function of the high voltage applied to the straws and gas amplifi-cation factors ranging from 1 to 7 × 105, were determined using a current measurement techniquefor various gas mixtures (see table 1). The gas gain A was determined from the ratio I/I0, where Iand I0 are the measured currents for a constant flux of X-ray photons for the applied voltage and forthe ionisation chamber regimes (the HV value when all ionisation is collected on the anode withoutgas gain), respectively. All measurements have been performed at atmospheric pressure. In orderto eliminate space charge effects, the measured current was kept below 1 nA. The accuracy of gasgain measurements is limited by the currents measurements and do not exceed 5%. The resultsshown in figures 5a and 5b are for the full set of studied gas mixtures.

Table 1 shows the maximum safe operating gas gain for all studied gas mixtures. Above thisvalue, the probability of self quenched (limited) streamers (SQS) becomes significant which leadsto an increase of discharge probability.

Table 1. Maximum gas gain for a safe straw operation.

Gas mixture Highest safe gas gain

Ar/CO2 80/20 4.9×104

Ar/CF4/CO2 70/20/10 2.4×104

Ar/CO2/N2 80/10/10 3.7×104

Xe/CO2 70/30 7.4×104

Xe/CF4/CO2 70/20/10 7.2×104

Xe/CO2/O2 70/27/3 9.8×104

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200 400 600 800 1000 1200 1400 1600 1800Voltage [V]

100

101

102

103

104

105

106G

as

gain

[-]

Ar/CF4 /CO2 70/20/10

Ar/CO2 80/20

Ar/CO2 /N2 80/10/10

(a) Ar-based gas mixtures.

0 500 1000 1500 2000Voltage [V]

10-1

100

101

102

103

104

105

106

107

Gas

gain

[-]

Xe/CF4 /CO2 70/20/10

Xe/CO2 70/30

Xe/CO2 /O2 70/27/3

(b) Xe-based gas mixtures.

Figure 5. Gas gain as the function of voltage applied between anode and cathode of straws for studiedmixtures, p=1030 hPa, T=293K.

The energy resolution (FWHM) has also been measured as a function of the applied voltage.The results obtained are presented in figure 6a, 6b and table 1. Energy resolution worsens close tothe gas gain safety limit. In order to ensure maximum stability of the GGSS, the working point waschosen to be slightly below the nominal TRT gas gain of 2.5 × 104.

1100 1200 1300 1400 1500 1600Voltage [V]

16

18

20

22

24

26

28

30

32

34

Energ

y r

eso

luti

on [

%]

Ar/CF4 /CO2 70/20/10

Ar/CO2 80/20

Ar/CO2 /N2 80/10/10

(a) Ar-based gas mixtures.

1300 1400 1500 1600 1700 1800 1900Voltage [V]

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Energ

y r

eso

luti

on [

%]

Xe/CF4 /CO2 70/20/10

Xe/CO2 70/30

Xe/CO2 /O2 70/27/3

(b) Xe-based gas mixtures.

Figure 6. Energy resolution for 5.9 keV 55Fe X-Ray line as a function of anode voltage for different gasmixtures.

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4.2 Gas gain temperature dependence

The electron amplification process near the wire is very sensitive to the density of the workinggas and thus to both temperature and pressure variations. A temperature change of ∆T = 1 K at293 K causes a change of gas pressure ∆p = 3.53 hPa at a pressure of p = 1035 hPa. While boththe TRT and the GGSS operate at atmospheric pressure, gain variations caused by temperaturedifferences can be significant and are compensated by applying HV corrections for each TRT regionseparately. The influence of temperature on the gas gain was studied in straw tubes from 15C to45C for the 80/20 Ar/CO2 gas mixture and the TRT baseline gas mixture (Xe/CO2/O2 70/27/3). Aspecial set-up which provided a thermal stability of 0.1 Cwas used for the temperature dependencestudy. The results of this study are presented in figures 7a and 7b which show the high voltageadjustment needed to compensate the temperature differences. As seen in figure 7, the temperaturedependence of peak position (and thus the gas gain) can be accurately approximated by straightline. The thermal coefficient of the gas gain (pulse height) is -1.54V/C for the 80/20 Ar-based gasmixture and -2.13V/C for the TRT baseline mixture Xe/CO2/O2 (70/27/3).

5 0 5 10 15 20 25 30Temperature difference [ C]

50

40

30

20

10

0

10

Volt

age d

iffe

rence

[V

]

Ar/CO2 80/20

Linear fit

(a) Ar/CO2 80/20 gas mixture.

5 0 5 10 15 20 25 30Temperature difference [ C]

60

50

40

30

20

10

0

10

Volt

age d

iffe

rence

[V

]

Xe/CO2 /O2 70/27/3

Linear fit

(b) Xe/CO2/O2 70/27/3 gas mixture.

Figure 7. Voltage adjustment needed to keep a constant gas gain as a function of a temperature change (thelower temperature was 15 C).

5 Design of the Gas Gain Stabilisation System

5.1 Hardware

The TRT GGSS consists of:

• two reference detectors with eight straws (15 cm in length) each enclosed in a brass box,

• 16 preamplifiers — one for each straw,

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• three high voltage power supplies and three DAC’s for their control (12 channels in total),

• two analogue signal multiplexers,

• two spectroscopic amplifiers,

• two multichannel analysers (MCA),

• and a control PC.

The straws are mounted inside a gas tight brass box that is equipped with gas inlet andoutlet manifolds which supply the straws with active gas in parallel. The straws (the cathodes) areconnected to the ground and the central wires (the anodes) connected to individual high voltage linesand preamplifiers. The straws run at a slightly lower voltage (50V) than the detector to minimiseradiation induced ageing. There are two PT1000 sensors measuring active gas temperature withaccuracy of ∼0.1. Each box has a gas tight Kapton window in its cover separating the strawsand 55Fe source. The signals generated by the straws are preamplified, then multiplexed to thespectroscopic amplifier, before finally being sent to a Multi Channel Analyser (MCA) for analysis.Because of the critical role of the GGSS in the TRT operation, every component has at leastone hot spare. During normal operation only one box is used and a second fully-equipped boxprovides a redundant backup in case of problems. A simplified schematic view of all major GGSSmodules and their interconnections are shown in figure 8. Tomonitor all critical working parameters(temperatures, gas flow, etc.) an Embedded Local Monitor Board (ELMB) is used [11]. It has 64input analogue channels from which information is send to the DCS for long term storage to allowhistorical trend monitoring.

HV

HV

Amp-lifier

Amp-lifier

MUX

MCA

MCA

DAC

DAC

Sof

twar

e

Straw 1

Straw 2

Straw 3

Straw 4

Straw 5

Straw 6

Straw 7

Straw 8

Preamplifier

Preamplifier

Preamplifier

Preamplifier

Preamplifier

Preamplifier

Preamplifier

Preamplifier

PC

Figure 8. Block diagram of the GGSS; Straw (1-8) — four stabilisation, two monitored and two spares;Preamplifier — charge sensitive, home made; HV — high voltage power suppliers CAEN model N472;Amplifier — ORTEC model 575A; DAC - digital analogue converters — MEN Mikro Elektronik GmbHmodel M37; MUX — analogue multiplexer — MEN Mikro Elektronik GmbH model M56; MCA —multichannel analyser — ORTEC model MCB — Trump PCI 2k.

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5.2 Software

Dedicated control software for the GGSS is integrated into the TRT DCS [13] and hence the globalATLAS control system. The core GGSS control application is written in C++ and runs as a serviceon a dedicated computer. The application communicates exchanging data and commands withDCS via the Distributed Information Manager (DIM) [12]. The software is responsible for theconfiguration and control of hardware components, data collection of measurements, and validationof calculated results. The outputs sent to DCS are the reference high voltage, the gas temperature,and their quality flags as well as any warnings or errors reported by the system.

A sketch of a simplified schema of software modules is shown in figure 9. The GGSS softwareuses signals from four “stabilisation” straws to compute a new value of the reference high voltage.The system collects spectra sequentially (using the multiplexer which routes the signals to theamplitude analyser). When the spectra of all four “stabilisation” straws are measured, a new value ofreference high voltage is computed as the mean value of the high voltages calculated for the correctlyworking stabilisation straws. The reference voltage is then applied to additional “monitored” strawsto check that its value results in the correct signal peak position. This cross-check is also usedfor longterm monitoring of the system behaviour. The whole procedure (which takes ∼10 min) isrepeated continuously as long as the GGSS is in operation. There are also two spare straws in theGGSS box which are to be used in case of problems with any of the “stabilisation” straws.

Start

Soft & hardware(re-)configuration

Straw Nconfiguration

Straw 1configuration

Straw Nmasurement

Stop

Straw 1masurement

Masurement 1veryfication

Sendingresults 1

Repeat fornext straw

Masurement Nveryfication

Sendingresults N

Figure 9. Simplified block diagram of software.

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6 Longterm system performance

For the entire LHC Run-1 (almost four years) the GGSS operated continuously during all periodsof ATLAS physics data taking.

6.1 Reference high voltage

The reference voltage is calculated using GGSS operation voltage and the known dependence ofthe gas gain on voltage. Under stable operating conditions HVref depends only on temperature andatmospheric pressure but will also react to gas mixture composition variation and changes to otherparameters. A normalised high voltage (HVnorm) value was introduced in order to monitor theoperation parameter stability. It is calculated using first principles using following formula:

HVnorm = (HVstart +ΩT (T − Tstart) +Ωp (p − pstart)) (6.1)

where HVnorm is the normalised high voltage as function of the temperature T and pressure p.The parameters in the formula are HVstart the reference high voltage (1483.8V ), Tstart the referencegas temperature (23.6C), pstart the reference pressure (976.5 hPa), ΩT the temperature coefficient(−2.13 V/C), andΩp the pressure coefficient (0.55 V/hPa ). The parameters shownwere measuredwhen the detector was running in optimal conditions at 2009-10-25 23:00:00. Figure 10 shows thelongterm behaviour of the reference (green line) and normalised (red line) high voltages as well astheir difference (blue line) where this difference should ideally be zero. Significant deviation fromzero usually indicates a problem with the gas composition. For instance after restarting the detectorin 2010 this difference reached a level of almost 40V caused by a miscalibration of the gas analyserwhich was used for active gas mixture control. This event demonstrates another important functionof the GGSS — monitoring the stability of the detector gas mixture which is critical to the properTRT operation.

2010-01-01

2010-05-01

2010-09-01

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1350

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V]

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Reference HVNormalized HV

2010-01-01

2010-05-01

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2012-09-01

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20

0

20

40

60

80

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age [

V]

HV Diff

Figure 10. Reference and normalised HV (left black scale/red and green lines) and their difference (rightblue scale/line) during GGSS performance over the three and a half year period from December 2009 toMarch 2013.

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6.2 Gas gain stability

The historic trend of the peak position for one of the stabilisation straws and for one of themonitoringstraws over a period of about 2 years is shown on figures 11 and 12. One sees an excellent stabilitywith the 55Fe line position maintained within ±1% for one of the gas gain stabilisation straws.The peak position for monitoring straw, figure 12, over long period of time is also stable within±1% except for some deviations that were caused by miscalibration of a high voltage power supply.Atmospheric pressure is a major factor of the the gas gain variation and figure 13 shows a variationof HVref (black curve) compared to the variation of atmospheric pressure (blue curve).

Figure 11. Peak position of 55Fe line for straw used for gas gain stabilisation (a highlighted region represents±1% range).

Figure 12. Peak position of 55Fe line for monitoring straw.

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950

960

970

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1000

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ssure

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age [

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2012-05-01

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2012-10-01

2012-11-01

2012-12-01

Date [-]

Figure 13. Longterm trends of crucial GGSS parameters: HVre f (black) and gas pressure (blue) in 2012;one can observe the expected strong correlation between the two.

In 2012, the luminosity of LHC reached a value where deposited ionisation in straws started tocause noticeable heating. Figure 14, shows TRT HV setting for one of the barrel HV region for a20 hour period during which there were two LHC fills producing significant numbers of collisions.From the top to bottom plot (a) shows the current in one barrel HV line which follows LHCinstantaneous luminosity, plot (b) shows temperatures measured by probes in one barrel region, plot(c) shows variation of the applied high voltages in the same barrel region and plot (d) shows thevalue of HVref received by DCS from the GGSS.

Electron separation from a hadron background is calculated from the fraction of straws crossedby a track with energy deposition above∼6 keV, the high level hit fraction. For electrons this fractionis about ∼25% and for pions it is ∼5%. In order to suppress pions, a cut on this parameter of about∼15% is applied so that a track with a high level hit fraction of more than 15% is considered tobe an electron track. This cut allows rejections of ∼95% of pions at a price of an ∼10% loss ofelectrons. The cut is a part of software analysis tool and is not expected to change during datataking. Any significant change of the gas gain would lead to a change of high level hit fraction onparticle track and hence to change in the fraction of pions which pass the cut along with change inelectron acceptance efficiency. A change of the gas gain by 10% leads to a change of high level hitfraction by 20%. Figure 15 shows a variation of the HL fraction during the 2011 running period.One sees that HL hit fraction varies by ∼3%, which corresponds to the gas gain variation of ∼1.5%within the TRT detector.

7 Conclusions

The excellent stability of the TRT during Run-1 to a large extent resulted from the TRT GGSSsystem maintaining nearly constant gas gain over the entire running period. Beyond maintainingstable gas gain in all parts of the TRT, the GGSS provides an efficient monitoring tool to detect

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(a)

(b)

(c)

Time [h]

HV

[V]

(d)

T [o C

]I [

µA

]

Figure 14. Current in one HV channel of the TRT (a); Trends of the temperature in one of the barrel TRTregions (b); High voltages applied to the same barrel region HVcor (c); the GGSS voltage HVref (followingthe atmospheric pressure) (d).

changes in the gas mixture composition not detected by other analysis devices of the TRT gassystem. It should be emphasised that during Run-1, GGSS was operating 100% of the time whenthe TRT and ATLAS were taking physics data. To the date of this publication, the GGSS hasnot had a single fatal failure proving the robustness of the system design and of the methods andcomponents chosen to build the system.

The GGSS has proven to be a key and very reliable component of the TRT detector meetingall design goals: system reliability confirmed by no fatal failures, accuracy of the gain stabilisationwell within the range of ∼1%. Moreover the GGSS is very sensitive to any active gas mixturechanges, therefore it acts as a fast and accurate “sensor” of problems related to the active gas.

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2011stDays since March,1

0 20 40 60 80 100 120 140 160

HL/

LL

0.062

0.064

0.066

0.068

0.07

0.072

0.074

0.076

0.078

0.08

Figure 15. Variation of HL hit faction on particle track during the 2011 running period.

The GGSS has contributed significantly to the global quality of the data acquired by the TRTdetector and hence by whole ATLAS experiment. This was achieved by keeping the gas gain inthe TRT detector within ∼1.5% stable range, therefore HL hit fraction variation was reduced toabout ∼3%, which significantly contributes to the high capability of the particle identification inthe ATLAS experiment.

After a successful major GGSS upgrade during the Long Shutdown 1 of the LHC, the systemwas ready for Run-2 physics data taking period and continues its operation.

Acknowledgments

This work was supported in part by the Polish National Science Centre, grant no. DEC-2013/08/M/ST2/00320. This work was supported in part by the Turkish Atomic Energy Authority.

References

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[3] ATLAS TRT collaboration, The ATLAS TRT end-cap detectors, 2008 JINST 3 P10003.

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[5] E. Hines for ATLAS collaboration, Performance of Particle Identification with the ATLAS TransitionRadiation Tracker, in Proceedings of Meeting of the APS Division of Particles and Fields(DPF 2011), Providence U.S.A. (2011) [arXiv:1109.5925].

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[6] T. Akesson et al., Straw tube drift-time properties and electronics parameters for the ATLAS TRTdetector, Nucl. Instrum. Meth. A 449 (2000) 446.

[7] K. Lantzsch et al., The ATLAS Detector Control System, J. Phys. Conf. Ser. 396 (2012) 1.

[8] A. Barriuso Poy et al., The detector control system of the ATLAS experiment, 2008 JINST 3 P05006.

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[11] B. Hallgren et al., The embedded local monitor board (ELMB) in the LHC front-end I/O controlsystem, at Electronics for LHC experiments, Stockholm Sweden (2001), pg. 325.

[12] C. Gaspar and B. Franek, Tools for the automation of large distributed control systems, IEEE Trans.Nucl. Sci. 53 (2006) 974.

[13] J. Olszowska et al., The ATLAS Transition Radiation Tracker (TRT) Detector Control System, inProceedings of ICALEPCS 2011, Grenoble France (2011), pg. 666.

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